If you are only planning on using it with your MacBook, then it really doesn't matter too much. The resolution of the MacBook isn't going to exceed what VGA can do. Also, the connection isn't as important as the monitor's specs. If it is a trashy monitor, then DVI isn't going to make it any better.
If you are thinking of keeping it around for a while, and maybe eventually using it on a computer with a serious graphics card, then you might think about getting a DVI monitor. I have noticed a slow shift to DVI over VGA.
Anyone, feel free to correct me if I am wrong.